One of the most convenient methods for deflecting the trajectory of a beam of charged particles is to use an interleaved comb of wires which form one type of a Bradbury-Nielson gate (BNG). As described in this application, such BNG may comprise two electrically isolated sets of equally spaced wires that lie in the same plane and alternate in potential. FIG. 1A summarizes the operation of this BNG. When no potential is applied to the wires relative to the acceleration energy of the charged particles, the trajectory of the charged particle beam is undeflected by the gate. To deflect the beam, bias potentials of equal magnitude and opposite polarity are applied to the two individual wire sets. Deflection produces two separate beam profiles, each of whose intensity maximum makes an angle α with respect to the path of the undeflected beam. In this manner it is possible to modulate or gate ion beams in a controlled fashion.
An extremely demanding application for these gates is Hadamard Transform Time-of-flight Mass Spectrometry (HT-TOFMS). In HT-TOFMS the ion beam is modulated with a pseudorandom sequence of on and off pulses by applying the corresponding modulation to a Bradbury-Nielson gate. Typical modulation rates are on the order of 10 to 20 MHz with modulation voltages of 10–50 V with respect to the voltage of ˜1 kV used to accelerate the ions. After the encoding sequence (usually a maximum length pseudorandom sequence) is applied, the ion packets created by the on/off modulation interpenetrate one another as they drift through the flight tube. The detected signal is a convolution of the mass spectra corresponding to these packets. Using knowledge of the applied encoding sequence, this signal is deconvoluted to yield a single mass spectrum This process is described in more detail in U.S. Pat. No. 6,300,626, which is incorporated by reference herein in its entirety.
The integrity of the deconvolution in HT-TOFMS is dependent on the profile of the applied pulses and the fidelity of the sequence felt by the ions. Ions that are improperly modulated because of spatial and energetic ambiguities at the gate will be observed as noise after deconvolution of the detector signal. If the error in modulation is time-invariant the noise appears as discrete peaks in the mass spectrum, called echoes. The position and the sign of the echoes depend on the nature of the modulation error.
The maximum achievable mass resolution of mass spectrometers that gate ions using a BNG is dependent on the duration of the pulses applied to the gate. Likewise, when using an ion gate for m/z selection, the mass resolution of the gate is dependent on how rapidly the gate can switch the beam on and off. The mass resolution of a Bradbury-Nielson gate is thus dependent on how fast the necessary voltage can be applied to the wires.
FIG. 1 depicts the three primary components of one possible set-up where the electronics associated with the BNG sat outside the instrument. The encoding sequence was generated by a system of shift registers, split into two inverse phases, and used to drive a push-pull amplifier (driver) to form a train of square pulses. In conventional implementations, these pulses traveled through significant lengths of transmission line to reach the BNG, which was housed inside the vacuum chamber of the MS. This complex set-up may be problematic for instrument performance and made repair of the BNG and the associated electronics unnecessarily time consuming.
Prior HT-TOFMS performance was ultimately limited by inaccuracies in the electronic sequence delivered to the BNG. Because of mismatched impedances between the driver and the BNG and the length of the transmission lines being used, such as would be possible in the set-up of FIG. 1, where the BNG driver is connected to the BNG by a transmission line, and the driver is situated outside the vacuum chamber, whereas the BNG is in the chamber. In such event, it is found that the square pulses were plagued by ringing, overshoot, slow settling rates, and mismatched voltages between the two wire sets and the instrument liner. These instabilities led to modulation errors, which in turn caused discrete echoes in the mass spectra and reduced the intensity of real peaks. In addition to decreasing sensitivity, echoes complicate the interpretation of mass spectra and reduce mass resolution by broadening real peaks, as echoes are common in the bins adjacent to real peaks. Because of the severe skewing, the frequency at which the modulation sequence was applied, and hence the maximum achievable resolution, was limited. It is, therefore, desirable to provide an improved system where the above described problems are alleviated or avoided. A detailed description of the problems encountered with the conventional design of a HT-TOFMS system is described in more detail on pages 278–280 of Effects of Modulation Defects on Hadamard Transform Time-of-flight Mass Spectrometry (HT-TOFMS), Kimmel, J. R.; Fernandez, F. M., Zare, R. N., 2003 American Society for Mass Spectrometry.